1. Field of the Invention
The present invention relates to a portable hydraulically powered or operated device adapted to work on a workpiece or workpieces, for performing a variety of operations such as processes involving plastic deformation of metals, clamping, caulking, drilling and cutting.
2. Discussion of the Prior Art
A relatively small-sized hydraulically powered device is widely used at job sites of plumbing and construction, for example. The device is either hand-held by the operator, or fixed on a work table. Generally, such a hydraulically powered device uses a single-acting cylinder to drive a working tool. The cylinder is hydraulically connected to a suitable hydraulic pump, so that a pressurized fluid is fed to a pressure chamber to move a piston and a piston rod, whereby the working tool is driven to achieve a desired operation on the workpiece. The piston is retracted to its home or original position by a return spring when the operation by the working tool is completed.
To retract the piston to the home position, it is required to turn off the hydraulic pump, and return the pressurized fluid from the pressure chamber of the single-acting cylinder to the hydraulic pump, by opening a fluid passage between the cylinder and the pump. Conventionally, a manually operated valve is provided in the fluid passage, so that the pressurized fluid is returned to the pump by opening this valve. After the piston is returned to the home position, the valve is operated to the closed position. Thus, the operation of the valve requires the operator to release the hand which has been engaged in holding the device. This may cause an inconvenience in performing the desired operation in the intended manner, and may reduce the working efficiency.
In view of the above drawback, a portable hydraulically operated device incorporating an automatically operated return valve is proposed as disclosed in laid-open Publication No. 53-47269 of examined Japanese Utility Model Application. This return valve is associated with a piston such that the return valve operates to open and close a fluid passage formed through the piston. The return valve is adapted to return the pressurized fluid from the pressure chamber to retract the piston to its home position when the piston has reached its stroke end. In this hydraulically operated device, however, the piston cannot be retracted before the stroke end of the piston is reached, that is, when it is desired to stop an operation on the workpiece for some reason or other, for example, when the operator finds an inadequate setup of the working tool or workpiece or some trouble after the working operation is started. Thus, this type of return valve is not practically satisfactory.
Usually, the hydraulic pump used for such a hydraulically operated device as described above is manually operated by reciprocating the operating lever several times. This pump is laborious and requires a relatively ample working space. Recently, the manually operated pump is replaced by an electrically operated pump. An example of such a hydraulic pump is indicated at 380 in FIG. 36, wherein the pump 380 is incorporated in a power unit 382 which includes, a reservoir 381, valves and other hydraulic components, as well as the pump. In this case, a single-acting cylinder 383 of a hydraulically operated device is generally connected to the power unit 382 through a relatively long hose 384. Further, a solenoid-operated valve 385 incorporated in the power unit 382 is controlled by a signal fed through a control cable. Thus, the control cable, and the hydraulic hose 384 (which is relatively heavy and inflexible) must extend between the power unit 382 and the working site of the device. Accordingly, the working site is limited, and the provision of the solenoid-operated valve 385 and the electric control switch for this valve increases the cost of the device as a whole. Further, the hydraulically operated device of the type shown in FIG. 36 is not suitable for performing a clamping operation, for example, for clamping workpieces to be welded together. More specifically, the power unit 382 employs a relief valve 387 disposed between the single-acting cylinder 383 and a check valve 386 near the delivery port of the pump 380. When the pump 380 is held off while maintaining the clamping pressure, the pressurized fluid in the system may leak through a minute clearance at the valve seat of the relief valve 387. To avoid this fluid leakage, the pump 380 must be kept operated during a welding operation on the clamped workpieces. This causes an increase in the working fluid and consequent deterioration of the fluid, and results in an increased energy consumption.